I just started regularly using the current trunk builds of Thunderbird and Seamonkey, and was surprised by the fact that
I was unable to open an mp3 file that was included either as an attachment or as inline content.
Not sure when this started, but surely is bothersome to have to first save the file before being able to open it.
Here is a snapshot: http://img101.imageshack.us/img101/6385/mp3la6.gif
I have looked at the discussion for https://bugzilla.mozilla.org/show_bug.cgi?id=315536 for MIME type of
"application/octet-stream" and of course, the mail content contains no such type.
Here is an example of a multipart related mail with an included mp3 file:
--------------010306060708080008030904
Content-Type: audio/mpeg;
  name="Eternals -  Babalus's Wedding DayFinal.mp3"
Content-Transfer-Encoding: base64
Content-ID: <part1.00030607.05030...@gmail.com>
Content-Disposition: inline;
  filename="Eternals -  Babalus's Wedding DayFinal.mp3"
It seems that just by virtue of the file extension, no option to play the file is offered.